This invention relates to an oil shale process, and more particularly, to a process for retorting oil shale and purifying and recycling effluent oil shale retort water.
Researchers have now renewed their efforts to find alternative sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sand by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily useable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained ih oil shale deposits in the United States with almost 60 percent located in the rich Green River oil shale deposits of Colorado, Utah and Wyoming. The remainder is contained in the linear Devonian-Mississippian black shale deposits which underline most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis for these vast resources.
Generally oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as oil shale retort water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
In order to obtain high thermal efficiency in retorting, carbonate decomposition should be minimized. Carbonate decomposition consumes heat, lowers thermal efficiency and decreases the heating value of off gases. Colorado Mahogany zone oil shale contains several carbonate materials which decompose at or near the usual temperature attained when retorting oil shale. Typically, a 28 gallon per ton oil shale will contain about 23 percent dolomite (a calcium/magnesium carbonate) and about 16 percent calcite (calcium carbonate) or about 780 pounds of mixed carbonate minerals per ton. Dolomite requires about 500 BTU per pound and calcite about 700 BTU per pound for decomposition, a requirement that would consume about 8 percent of the combustible matter of the shale if these minerals were allowed to decompose during retorting. Saline sodium carbonate minerals also occur in the Green River formation in certain areas and at certain stratigraphic zones.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen and oil shale, known as retorting, to form liberated hydrocarbons, can be done in in situ retorts underground or in surface retorts above ground. In principle, the retorting of oil shale comprises heating the oil shale to an elevated temperature and recovering the vapors and liberated effluent. However, as medium grade oil shale yields approximately 20 to 25 gallons of oil per ton of shale and significant quantities of oil shale retort water, the expense of materials handling and retort water treatment is critical to the economic feasibility of a commercial operation.
In in situ retorts, a flame front is continuously or intermittently passed through a bed of rubblized oil shale to liberate shale oil, off gases and oil shale retort water. There are two types of in situ retorts: true in situ retorts and modified in situ retorts. In true in situ retorts, the oil shale is explosively rubblized and then retorted. In a modified in situ retort, some of the oil shale is removed before explosive rubblization to create a cavity or a void space in the retorting area. A cavity provides extra space for rubblized oil shale. The oil shale which has been removed is conveyed to the surface and retorted above ground.
After an in situ retort is burned, the volume of spent shale within the retort is diminished and commonly does not adequately support the overlaying structure. This lack of support can lead to surface subsidence. Furthermore, spent in situ retorts can cave in if a new in situ retort is formed in an underground area closely adjacent the spent in situ retort. In order to increase the structural strength and integrity of the spent in situ retort, the spent retort can be backfilled with a slurry of spent oil shale and water.
In situ retorting and backfilling are described in U.S. Pat. Nos. 1,913,395; 1,191,636; 2,418,051; 3,001,776; 3,586,377; 3,434,757; 3,661,423; 3,951,456; 4,007,963; 4,017,119; 4,120,355; 4,126,180; 4,131,416; 4,133,380; 4,149,752; 4,194,788; 4,231,617 and 4,243,100 as well as in the patent application of John M. Forgac and Gerald B. Hoekstra for In Situ Retorting of Oil Shale with Pulsed Combustion, Ser. No. 265,287, filed May 20, 1981, which is assigned to the assignee of the present application.
In surface retorting, oil shale is mined from the ground, brought to the surface, crushed, sized and placed in a surface retort above ground where it is contacted with a hot heat transfer carrier, such as hot spent shale, sand, ceramic balls, metal balls or gases, or mixtures thereof for heat transfer. The resulting high temperatures cause the light hydrocarbon gases, shale oil and oil shale retort water to be liberated from the oil shale leaving a retorted, inorganic material and carbonaceous material such as coke. The carbonaceous material can be burned by contact with oxygen at oxidation temperatures to recover heat and to form spent shale relatively free of carbon. Spent oil shale which has been depleted in carbonaceous material is removed from the retort and reheated for use as heat carrier material or discarded. The liberated hydrocarbons and combustion gases are dedusted in cyclones, electrostatic precipitators, filters, desalters, water spray scrubbers or pebble beds.
Some well known processes of surface retorting are: N-T-U (Dundas Howes retort), Kiviter (Russian), Petrosix (Brazilian), Lurgi-Ruhrgas (German), Tosco II, Galoter (Russian), Paraho, Koppers-Totzek, Fusham (Manchuria), Union Rock Pump, gas combustion and fluid bed. Process heat requirements for surface retorting processes may be supplied either directly or indirectly.
The Lurgi-Ruhrgas process and modifications thereof are described in U.S. Pat. Nos. 3,655,518; 3,703,442; 3,962,043; 4,038,045 and 4,054,492 and in the articles by Marnell, P., entitled Lurgi-Ruhrgas Shale Oil Process, published in Hydrocarbon Processing, pages 269-271 (September 1976); Schmalfeld, I. P., The Use of the Lurgi-Ruhrgas Process for the Distillation of Oil Shale, Volume 70, Number 3, Quarterly of the Colorado School of Mines, pages 129-145 (July 1975); Rammler, R. W., The Retorting of Coal, Oil Shale and Tar Sand by Means of Circulated Fine-Grained Heat Carriers as a Preliminary Stage in the Production of Synthetic Crude Oil, Volume 65, Number 4, Quarterly of the Colorado School of Mines, pages 141-167 (October 1970), and at pages 81-85 of the Synthetic Fuels Data Handbook by Cameron Engineers, Inc. (Second Edition 1978).
The Tosco II process and modifications thereof are described in U.S. Pat. Nos. 3,003,894; 3,034,979 and 3,058,903 and at pages 85-88 of the Synthetic Fuels Data Handbook.
The Union Rock Pump retorting process is described in U.S. Pat. Nos. 2,501,153; 2,640,019; 2,875,137; 2,881,117; 2,892,758; 2,954,328; 2,966,446; 2,989,442; 3,004,898; 3,039,939; 3,058,904; 4,003,797; 4,043,897 and 4,162,960 and at pages 95-100 of the Synthetic Fuels Data Handbook.
Various fluid bed retorting processes are described in U.S. Pat. Nos. 4,087,347; 4,125,453; 4,133,739; 4,157,245 and 4,199,432.
The Fusham process is shown and described at pages 101-102, in the book Oil Shales and Shale Oils, by H. S. Bell, published by D. Van Norstrand Company (1948). The other processes are shown and described in the Synthetic Fuels Data Handbook.
Significant quantities of oil shale retort water are produced during retorting. Oil shale retort water is laden with suspended and dissolved impurities, such as shale oil and oil shale particulates ranging in size from less than 1 micron to 1,000 microns and contain a variety of other contaminants not normally found in natural petroleum (crude oil) refinery waste water, chemical plant waste water or sewage. Oil shale retort water usually contains a much higher concentration of organic matter and other pollutants than other waste waters or sewage causing difficult disposal and purification problems.
The quantity of pollutants in water is often determined by measuring the amount of dissolved oxygen required to biologically decompose the waste organic matter in the polluted water. This measurement, called biochemical oxygen demand (BOD), provides an index of the organic pollution in the water. Many organic contaminants in oil shale retort water are not amenable to conventional biological decomposition. Therefore, tests such as chemical oxygen demand (COD) and total organic carbon (TOC) are employed to more accurately measure the quantity of pollutants in retort water. Chemical oxygen demand measures the amount of chemical oxygen needed to oxidize or burn the organic matter in waste water. Total organic carbon measures the amount of organic carbon in waste water.
Over the years, a variety of methods have been suggested for purifying or otherwise processing oil shale retort water. Such methods have included shale adsorption, in situ recycling, electrolysis, flocculation, bacteria treatment and mineral recovery. Typifying such methods and methods for treating waste water from refineries and chemical and sewage plants are those described in U.S. Pat. Nos. 2,948,677; 3,589,997; 3,663,435; 3,904,518; 4,043,881; 4,066,538; 4,069,148; 4,073,722; 4,124,501; 4,178,039; 4,121,662 and 4,289,578. These prior art methods have met with varying degrees of success.
It is therefore desirable to provide an improved process for retorting oil shale and purifying oil shale retort water.